This invention relates to a novel and improved thread cutting mechanism in a lock stitch sewing machine in which a stationary blade fixedly secured to the machine framework below a throat plate and a reciprocally movable blade cooperate with each other in cutting an indefinite length of thread connected to a workpiece being sewn and more particularly, to improvements in and relating to an operation device for the thread cutting mechanism so as to perform positive thread cutting.
Before in the conventional thread cutting means illustrated in FIGS. 2(A)-(D) the reciprocal or swinging blade 2 pivotable about the vertical shaft 1 is formed at its leading edge with a thread handling portion 2a. This portion separates the upper and lower threads Ta and U connected to the workpiece being sewn from the upper thread portion Tb connected to the sewing needle during the counter clockwise movement of the blade from the FIG. 2(A) to the FIG. 2 (B) position. At its trailing edge the blade is formed with a thread arresting portion 2b where said threads Ta and U are arrested when the clock-wise blade movement from the FIG. 2(B) to the FIG. 2(C) position occurs. At the outer end face the blade is formed with a knife portion 2(c) where said arrested threads Ta and U are cut by the relative movement between said knife portion and the fixed blade 3.
During the counter clockwise reciprocal movement of the blade the upper and lower threads Ta and U are initially engaged by the portion 2a then frictionally move along the arcuate guide portion 2d until they slide off to a position adjacent said thread arresting portion. With thick or stiff threads however there is sufficient tension on said threads to create a high amount of friction between them and the trailing edge of the portion 2d such that the yarn is not released therefrom as shown in FIG. 2(D). They thus can't be cut by said relative blade movement when the movable blade 2 assumes the FIG. 2(C) position.